1. Field of the Invention
The present invention relates to polyvinyl alcohol binder fibers capable of melting under low-energy drying condition and capable of giving paper and nonwoven fabrics of high strength. The present invention also relates to paper and nonwoven fabrics comprising the fibers.
2. Description of the Related Art
At present, polyvinyl alcohol (hereinafter abbreviated to PVA) fibers are used as binder fibers in papermaking, as they are soluble in water and their adhesiveness is high. The adhesiveness of PVA binder fibers is high, and this is because the fibers swell in water where they have dispersed in a step of papermaking from them, and may therefore well melt under heat in a step of drying them, and they crystallize while being dried.
Heretofore when PVA fibers are used in producing paper or nonwoven fabrics, a thermal drum-type Yankee drier is generally used in the step of drying them. The Yankee drier generates a large quantity of heat for drying, and therefore when PVA binder fibers are dried therein, they may well melt and express high adhesiveness. However, with the recent tendency in the art toward efficient drying and improved productivity, air-through driers and the like have become much used in many cases, but they are problematic in the following point. When air-through driers are driven for drying therein, they have a short drying time and generate a small quantity of drying heat, and therefore ordinary PVA binder fibers could not well melt while dried therein, and, as a result, the dried fibers could not express sufficient adhesiveness.
To solve the problem as above, various methods have been employed. For example, PVA resin having a low degree of saponification is used for the starting material; or an ionic functional group, for example, a cationic group such as carboxyl group, sulfonic acid group, silyl group or quaternary ammonium group is introduced into PVA resin to thereby improve the solubility of the resulting resin. Specifically, the degree of saponification of PVA resin is lowered so as to increase the solubility of the resin, and the degree of polymerization of PVA resin is lowered so as to increase the solubility of the resin, and various methods for these have been proposed (for example, refer to Patent References 1, 2). Another technique has also been proposed, which comprises introducing a silyl group or an ethylene group into PVA resin to thereby increase the solubility and the adhesiveness of the resin (for example, refer to Patent References 3, 4, 5, 6).
In JP-A 51-96533, JP-A 54-96534, JP-A 60-231816, JP-A 4-126818, JP-A 58-220806, and JP-A 2003-27328, modification of PVA resin is essentially investigated for attaining increased adhesiveness of binder fibers. In these, however, the binder fibers are produced through melt spinning or wet spinning through spinnerets with round orifices, and therefore, the cross-sectional profile of the fibers is roundish or cocoon-shaped, and the cross-section circularity of the fibers, which is calculated according to the calculation formula to give a cross-section circularity from a cross-section profile of fibers, is 35% or more. As a result, the binder fibers obtained in JP-A 51-96533, JP-A 54-96534, JP-A 60-231816, JP-A 4-126818, JP-A 58-220806, JP-A 2003-27328 are problematic in that, though they could be well adhesive when dried under high-energy drying condition such as a thermal drum-type Yankee drier system, they could not be well adhesive when dried in high-speed drying such as a hot air drying system or under low-temperature/low-energy drying condition such as in a multi-cylinder system.